Vision is based on photoreceptors sending electric impulses from defined regions of the retina to neuronal cells that pass through two lateral geniculate bodies and relay the impulses to defined regions in the primary visual cortex. Stereopsis is achieved by visual input from two eyes, which create slightly different images due to their differing viewing angles. In the absence of stereopsis, other cues to depth, including parallax, perspective, depth from movement, and occlusion, can be used to compensate and retain some depth perception.
The mechanisms employed by the brain to merge the images received from the two eyes are not fully understood. Convergence of the binocular input has been shown to occur beyond the geniculate bodies and to be a complex process that involves the primary visual cortex and other areas of the brain. Processes employed by the brain to deal with incongruent input from the two eyes are called adaptation processes. For example, studies in strabismus have shown that input from one eye that is weaker or out of register with input from the other eye, if not corrected early in infancy, can lead the brain to develop in a way that ignores the signal input from the weak or misdirected eye. Adaptation processes can be used therapeutically to “train” the eye to adjust to reorganized or distorted images.
Vision impairment is a major health concern with increasing numbers of aging and vision-impaired subjects. Age-related macular degeneration (AMD) and glaucoma are two eye diseases that cause visual disability and blindness. The macula comprises only four percent of the retinal area and contains the fovea, an area responsible for central, high resolution vision. AMD is the leading cause of visual disability and blindness in the population over 65 years of age in the United States and other Western countries. It affects 11% of the population of 65-74 year olds and 28% of the population over 74 years old.
AMD is generally divided into two forms, non-exudate, atrophic or dry AMD and neovasclar, exudate or wet AMD. Dry AMD, which constitutes more than 80% of all AMD cases, is characterized by drusen retinal pigment epithelium (RPE) atrophy, and cone photoreceptor degeneration. In geographic atrophy, the most severe form of dry AMD that accounts for >20% of blindness in North America, the central vision is severely affected but the peripheral vision in most subjects is relatively normal. No effective treatment is currently approved for dry AMD.
Glaucoma is a disease that causes a gradual degeneration of ganglion cell axons that make up the optic nerve and the retinal nerve fiber layer (RNFL). Vision loss usually begins in the retinal periphery and slowly progresses as the nerve cells die. According to National Eye Institute, among the U.S. population 40 years and older, there are 2.218 million open-angle glaucoma subjects (290,000 between the ages of 40 and 49). Owing to the rapid aging of the U.S. population, this number will increase to more than 3 million by 2020.